Vacuum tube voltmeter circuit



732/77: Ina/S (7 1942- I. ANDERSON 2,294,065

VACUUM TUBE VOLTMETER CIRCUIT I Filed Nov. 2, 1959 19.0. v Inez/7' Meter'/N VENTOB Ealrbl finds/1:01p,

firmelvfw i Patented Aug. 25, 1942 PATENT" OFFICE 2,294,065 VACUUM TUBEVOLTMETER CIRCUIT Earl I. Anderson, Bayside, N.

Corporation of America, a

aware I, minor to Radio corporation of Del- Appiication November 2,1939, Serial No. 302,483 7 2 Claims. (Cl. 111-95) My present inventionrelates to vacuum tube voltmeter circuits adapted for measuringalternating current voltages, and more particularly to animproved'voltmeter adapted for measuring alternating current voltages ofrelatively small magnitudes.

One of the main objects of my present invention is to provide analternating current vacuum tube voltmeter oi the diode rectifier type,wherein electronic device is utilized for substantially eliminating theefiect of contact potential oi the rectifier diode.

Another important object of this invention is to provide in operativeasscciation'with the rectifier diode of an alternating current vacuumtube voltmeter, a second diode which functions to balance out the efiectof the contact potential or the voltmeter diode.

Still another object of this invention is to provide an alternatingcurrent voltmeter of the type including a diode rectifier, a loadresistor and a meter device, a second diode being arranged in buckingrelation to the voltmeterdiode thereby to eliminate any indication inthe meter device which arises by virtue of the contact potential of thevoltmeter diode.

And still other objects of this invention are to improve generally theemciency and reliability of alternating current vacuum tube voltmeters,and

more especially to provide an alternating current voltmeter of the dioderectifler'type which is not I range desired to be only ei'licient andreliable in operation, but is I economically manufactured and assembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in-the appended claims; the inventionitself, however, as to both its organization and method of operation,will best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into efiect.

Referring nowtothe accompanying drawing; there is shown a vacuum tubevoltmeter of well known and conventional type. Such a voltmeter usuallycomprises an electron discharge device functioning as arectifier, a loadresistor and a voltage indicating meten. An electron discharge device ofthe diode type is commonly employed at the present time, because thediode device provides diode rectification of the alternating currentsimpressed upon the input terminals of the voltmeter. There are manyadvantages secured when u diode rectification. Among these advantages isthe very important one oi of'the alternating current input relativelysmall, yet if i denotes the anode of the voltmeter diode, and

the numeral 2 designates the cathode which may be of the indirectlyheated type. The anode i is connected by condenser 8 and lead 3 to oneterminals, while the opposite input terminal is connected to the cathode2 through lead 6. The direct current return path is through the resistor6 and the meteri. 2,, a M

The meter 5, may be of any well known type, and isschematicallyrepresented since; those skilled in the art are fullyacqualntedwiththe construction of meters commonly employed, in vacuumtube voltmeters. The resist'or. 6 may be provided with a plurality pfspacedp tgps long the length thereof so as to providea multl; rangeinstrument. arm i is provided so that the resistor may be adiusted inmagnitude depending on the voltage indicated.

The diode i--2 in'the absence of alternating current input to theinstrument possesses a small current flow which is due to a phenomenonknown as contact potential. Briefly, contact potential arises by virtueof the initial speed of the electrons leaving the heated cathode 2, andbeing collected by the cold electrode i. Although the current flowcaused by contact potential is the valueof resistance 8 employed issmall, or the range of the meter is low, the current flow due to contactpotential can be very large compared with that resulting from thealternating current voltage to be measured. In other words, if there isimpressed upon the input terminals of the voltmeter an alternatingcurrent voltage which is of the order of magnitude of the voltagedeveloped across resistor 8;

' for an initial indication due to the current iiow expedient is notreliable since the battery degencaused by contact potential of thediode. It has been attempted in the past to counteract the effect ofcurrent flow due to contact potential by using a small battery inbucking" relation to the contact potential current. However, such ancrates appreciably in a very short time, and the user of the voltmetermust be continually checking the meter. It is In such case anadiu'stable contact essential to provide a means for substantiallyeliminating the efiect oi! the current due to contact potential withoutthe voltmeter user having to concern himself about the compensatingdevice for long periods of time.

In my present invention this long-range compensating'eflect is providedin a simple and reliable manner by connecting across the meter a secondelectron discharge device. Specifically the latter is a diode whichcomprises the anode II and the indirectly heated cathode ll. ode II isconnected to the junction of meter 5 and resistor 6 through a path whichincludes the resistor i4 and lead l2. The anode l0 of the seconddiode isconnected by lead l3 to the low Cathpotential end 01' meter 5. The fixedend of adiustable arm 1 is connected to lead l2. The load resistor l4,if the voltmeter is of the variable range type, is provided with tapssimilar to the taps located on resistor 6, and the adjustable switch armI is provided for connecting the same magnitude of resistance in circuitwith diode Iii-ll as is connected in'circuit with diode l2.

The fixed end of switch I is connected to lead II. The dotted line I5indicates a mechanical control coupling between switch arms I and I sothat the resistance values of resistors 6 and [4 may be kept the same inorder to maintain the two direct currents due to the contact potentialsof both diodes oi the same magnitude. Since diode Ill-ll is connected inopposed relation to diode l-2 across meter 5, it will be clear thatthere will be no-fiow of direct current through meter 5 due to thecontact potentials of both diodes. If desired the section H of resistorl4 may be made independently adjustable so'that the direct currents, dueto the contact potentials of both diodes. may be kept exactly equal invalue should the contact potential of diode l0H vary somewhat due toaging, or due to its initial variation caused by manufacture.

It will then be seen that according to my invention the contactpotential of diode Ill-ll is utilized to generate a direct currentthrough resistor It which flows through meter 5 in polarity oppositionto the direct current resulting from voltage developed across resistor 6because of the contact potential of diode |-2. The two diodes may beindependent of each other, but preferably they are mounted within acommon tube envelope 2t, and the tube employed may be of the 61-16 type.In this way the range of meter proportionality is increased since theonly indications appearing on the scale on meter 5 will be those due tothe alternating current voltage impressed upon the input terminals ofthe voltmeter. Actual experience with a compensating voltmeter of thetype disclosed herein shows that complete compensation is bad forperiods as long as two years. The meter 5, in an actual experiment, was200 microamperes, full scale, while resistor 8 was a 10,000 ohm resistorand resistor II was variable from 5000 to 15,000 ohms. If both halves ofa 6H6 type tube are used for this purpose then there is no reason whythe compensation should not be eflective for as long as the life of thetube i self. The present compensation arrangement is particularlyadvantageous for 'measuring alternating current voltages over a range ofvalues which would be substantially affected by the contact potentialvalue of a conventional diode which may be from /2 to 1 volt.

While I have indicated and described a system for can-ying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of myinvention, as set forth in the appended claims.

WhatIclaim is:

1. In combination with an alternating current voltmeter circuit of thetype including a diode rectifier having a pair of alternating-voltageinput terminals, impedance means in circuit with said diode fordeveloping direct current voltage from rectified alternating voltage,and a visual voltage indicator in circuit with said voltage developingimpedance means,,an arrangement for balancing out the eflect of directcurrent voltage developed across said developing means by the direct,current due to the diode contact potential comprising a second diodehaving its electrodes y connected across said visual voltage indicator.

an impedance in circuit with solely said second diode and indicator fordeveloping the direct current voltage from the direct current flow inthe circuit of the second diode due to contact potential thereof, andsaid second direct current voltage being applied to said voltageindicator in polarity opposition to the voltage derived from the firstdiode contact potential said indicator being connected between thejunction of said impedance and said impedance means and one of saidterminals.

2. In combination with an alternating current voltmeter circuit of thetype including a diode rectifier having a pair of alternating voltageinput terminals, resistor means in circuit with said diode fordeveloping direct current voltage from rectified alternating voltage,and a visual voltage indicator in circuit with said voltage developingmeans, an arrangement for balancing out the efiect of direct currentvoltage developed across said resistor means by the direct current dueto the diode contact potential comprising. a second diode having itselectrodes connected across said visual voltage indicator, an impedancein circuit with solely said second diode and indicator for' the voltagederived from the first diode contact potential, said indicator beingconnected between the junction of said resistor and impedance and one ofsaid terminals, a single tube envelope housing the electrodes of bothsaid diodes, and means for adjusting theresistor and impedance of bothdiode circuits so that the resistive magnitudes in both circuits are thesame.

EARLYI. annnnsorf

